X.PC is a deprecated communications protocol developed by McDonnell-Douglas for connecting a personal computer to its Tymnet packet-switched public data telecommunications network. It was originally developed to enable connections up to 9600 baud. Unlike MNP, a competing standard proposed by Microcom, X.PC was placed in the public domain for royalty-free usage. MNP, on the other hand, required a $2,500 licensing fee.
X.PC is actually a subset of X.25, which is a CCITT standard for packet-switched networks. It is a full-duplex, asynchronous and error-correcting network protocol that supports up to 15 simultaneous channels. X.PC is defined as a link-level protocol on OSI. It maintains automatic error correction during any communications session between two or more computers.